Composite materials, such as fiber-reinforced composite materials, have become popular as a constituent of high-performance products and components that need to be lightweight, yet strong enough to take harsh loading conditions. Examples of such applications are aerospace components, including the tails, wings, fuselages, and propellers of aircraft; boats and other marine vessels; bicycle frames; and car bodies. Modern jet aircraft include fuselages composed largely of composites. Carbon fiber-reinforced polymers (CFRPs) are used in the fuselages of aircraft and space vehicles.
One type of composite material is a lay-up comprised of a honeycomb core sandwiched between two outer skin layers above and below the honeycomb core. The entire sandwich may be made of a material such as fiberglass, a CFRP, aluminum, Nomex (a trademark of E. I. Du Pont De Nemours and Company, Wilmington, Del.), or other materials. Further forms of composites may include foam cores.
The use of such composite materials in high-stress areas of vehicles, such as aircraft, necessitate that the materials in such stress areas be tested periodically to determine whether damage has been sustained, because damage may occur without being visibly apparent on the outer skin of the laminate sandwich. Damage may take the form of disbonding, that is, a separation between the core and the outer skin, and a crushed core. It is desirable to test the composite structure in situ for efficiency reasons, and further, to conduct tests of the material without invasive measures such as scraping or drilling of holes.
Consequently, non-destructive testing systems have been developed. One such system, disclosed in Published U.S. Patent Application Pub. No. 2013/0338941 titled METHOD AND APPARATUS FOR DEFECT DETECTION IN COMPOSITE STRUCTURES by Lin et al., the contents of which are incorporated herein by reference, discloses a method and apparatus for defect detection in composite structures. That system and method utilize a “pitch-catch” probe connected to a control box that is controlled by a laptop computer or other computing device. The pitch-catch probe includes two transducers, which may take the form of probe tips spaced approximately one-half inch apart: a first transducer that transmits a sound signal, which may be sonic or ultrasonic, to and through the surface of the composite material to be tested, and a second transducer that receives the sound or sound signal that has traveled through the part. Changes in the structure of the part (such as defects) change how the sound travels through the part and can be detected by examining the amplitude, phase, and frequency of the received waveform. The alteration of the frequency, phase, and amplitude of the sound signal is processed by the control box to determine the presence of a defect. Variations in the received sound signal may be matched with a library of known signals for that particular composite material being tested, so that the received signal can be used to determine a type of defect in the composite material.
Traditionally, pitch-catch probes have been driven at a single frequency. However, it is known that different flaws are detected better when using different sound frequencies, but different flaw configurations have different natural resonant frequencies. Multiple discrete probe drive frequencies or a continuously swept sine wave probe drive are used to generate sound at multiple frequencies to increase the probability of finding any flaw in the part. A swept frequency probe drive transmits a plurality of sound waves at different frequencies. Common frequencies are between 10 kHz and 30 kHz, but other frequencies are also used. The probe drive frequency can be swept up or down.
Since the piezoelectric crystals in the pitch-catch probe generate and detect sound with different efficiency at different frequencies, a problem with swept frequency probes is that the appropriate instrument gain setting changes with frequency. When the gain is set to keep the high frequency signals on the screen of an associated display, the low frequency signals can no longer be seen, and appear as an almost flat trace (see FIGS. 4 and 5). In other cases, the converse may be true. Accordingly, the usefulness of the swept frequency probe drive on a pitch-catch bond tester is limited because the appropriate instrument gain setting varies with frequency. Since the gain may be set to keep high (or low) frequency signals on the screen, the low (or high) frequency signals cannot be seen even though they often have a higher signal-to-noise ratio. Consequently, there is a need for a non-destructive testing system and method that overcomes the shortcomings of such devices.